Method and apparatus for frequency tracking in a space time transmit diversity receiver

ABSTRACT

A system and method for obtaining a frequency error estimate representing the difference between a reference frequency and the frequency of a space-time transmit diversity signal is disclosed herein. The method includes taking the correlation of total sums, comprised of partial sums taken in defined first and second intervals, to represent the frequency error as the imaginary component of the correlation function.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from and is related to thefollowing prior application: “METHOD AND APPARATUS FOR FREQUENCYTRACKING IN A SPACE TIME TRANSMIT DIVERSITY RECEIVER”, U.S. ProvisionalApplication No. 60/273708, filed Mar. 6, 2001. This prior application,including the entire written description and drawing figures, is herebyincorporated into the present application by reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] This invention relates to the art of receiving a Space TimeTransmit Diversity (STTD) signal. In particular, this invention relatesto frequency tracking of an STTD signal. The invention finds applicationin a closed-loop automatic frequency control in wireless user equipment.The invention is particularly well suited for use in Personal DigitalAssistants, mobile communication devices, cellular phones, and wirelesstwo-way e-mail communication devices (collectively referred to herein as“wireless devices”). The invention provides utility, however, in anydevice that receives an STTD signal.

[0004] 2. Description of the Related Art

[0005] Space Time Transmit Diversity (STTD) reception is often mandatoryfor user equipment (UE), such as mobile communication devices, tooperate in a standard fashion with various wireless communication radionetwork sub-systems (RNS), such as base stations. For example, in the3rd Generation Partnership Project (3GPP) standard document No. 3GTS25.211 V3.1.1 (1999-12), it is clearly indicated that STTD reception ismandatory for UE.

[0006] The concept of STTD transmission is known to those of skill inthe art and involves the use of two transmit antennas at the RNSemploying a space time block coding, such as the example illustrated inthe block diagram of an STTD encoder of FIG. 1.

[0007] Although STTD transmission at an RNS is meant to be beneficial toreception at the UE, frequency tracking at the UE is complicated by STTDtransmission.

[0008] Typically, in non-STTD systems, UE tracks an RNS pilot signal inorder to control a local reference oscillator. The pilot signal isusually specifically designed in order to facilitate determining afrequency offset.

[0009] However, when the received signal from the base station is anSTTD signal, detection of the frequency offset from the received signalis more difficult. FIG. 2 illustrates a typical pilot modulation patterntransmitted by STTD. The symbol A is a complex number with real andimaginary parts. In this particular case, the symbols from antenna 1 arealways +A, while the symbols from antenna 2 are alternatively +A and −Awith pattern shown. One problem with this pattern is that the two signalcomponents can interfere with each other at the UE. Although notexplicitly illustrated, one of skill in the art will appreciate thatother patterns exist which present the same problem. Considering typicalpropagation conditions between RNS and UE, conventional methods forfrequency tracking do not have sufficient performance to enable reliablefrequency tracking. An alternative to frequency tracking is to rely onhighly stable frequency reference source in the UE. However, thisalternative is neither cost effective nor is it optimal from thereceiver performance perspective.

[0010] There is a need for a method and apparatus for detectingfrequency error between a frequency reference and a received STTD signalat UE. There is a further need for a method and apparatus that controlsthe frequency reference by tracking a received STTD signal at UE.

SUMMARY

[0011] It is an object of the present invention to obviate or mitigateat least one disadvantage of previous frequency discriminators for STTDsignals.

[0012] It is a particular object of the present invention to provide amethod and apparatus for detecting frequency error between a frequencyreference and a received STTD signal at UE. It is a further object ofthe present invention to provide a method and apparatus that controlsthe frequency reference by tracking a received STTD signal at UE.

[0013] This invention uses the statistical properties of symbolstransmitted in an STTD signal to efficiently remove the interferenceintroduced between the two STTD antennas at the UE. Removing theinterference provides a wide range for frequency error detection, whichincreases the control range, relaxes the requirement for frequencyreference accuracy, and eventually reduces UE cost.

[0014] In a first aspect, the present invention provides a method ofobtaining a frequency error estimate of the difference between areference frequency and the frequency of a space time transmit diversitysignal from first and second received sequences of symbols, transmittedrespectively by first and second antennae, where each sequence has twosets of first and second intervals, such that the contents of the secondinterval of the second received sequence are the additive inverse of thecontents of the first interval of the second received sequence, themethod comprising the steps of: receiving the first and second sequencesof symbols; calculating two sets of first and second partial sums as thesum of the contents of the first and second intervals, respectively, foreach set; calculating total sum functions for the first and second setsby summing the first and second partial sums for each set; calculating acorrelation function based on the total sum functions for the first andsecond sets; and extracting the frequency error estimate from thecorrelation function.

[0015] In an embodiment of the first aspect of the present invention thecorrelation function is calculated as a time average of the product ofthe first total sum function and the conjugate of the second total sumfunction. In other embodiments the received symbols are represented bycomplex numbers, and the step of extracting includes isolating theimaginary part of the correlation function as the frequency errorestimate.

[0016] The first and second intervals in each set can be adjacent, orthey can be interleaved with the first and second intervals of the otherset. The intervals can also be half or whole symbols in length.

[0017] In one embodiment, the step of calculating the total sum includesmultiplying the second partial sum for each set by −1, either inaddition to, or as a replacement of the original total sum step. In afurther embodiment the correlation of the two total sums are added tocreate a third correlation function from which the error can beextracted. In yet another embodiment there is included the step ofmultiplying the frequency error estimate by the average of asignal-to-noise-ratio of the received sequences.

[0018] In another embodiment a method of controlling the referencefrequency to match the frequency of the STTD signal is also provided,using the above described steps, and further comprising the step ofaltering the reference frequency based on the frequency error estimateto minimize the difference between the reference frequency and thefrequency of the space time transmit diversity signal.

[0019] In a second aspect of the present invention there is provided anapparatus having a frequency discriminator for obtaining a frequencyerror estimate of the difference between a reference frequency and thefrequency of a space time transmit diversity signal from first andsecond received sequences transmitted respectively by first and secondantennae, and received by a receiving antenna, where each sequence hastwo sets of first and second intervals, of equal length, such that thecontents of the second interval of the second received sequence are theadditive inverse of the contents of the first interval of the secondreceived sequence, the frequency discriminator comprising: a memory,operatively attached to the receiving antenna for storing the contentsof the first and second sequences; interval defining means, operativelyattached to the memory to receive the first and second sequences ofsymbols, for dividing the received sequences into sets of first andsecond intervals; partial sum adding means, operatively attached to theinterval defining means to receive the contents of first and secondsequences during the two sets of first and second intervals, forcalculating two sets of first and second partial sums as the sum of thecontents of the first and second intervals respectively for each set;total sum adding means, operatively attached to the partial sum addingmeans to receive the two sets of first and second partial sums, thetotal sum adding means for calculating total sum functions for the firstand second sets representing the sum of the first and second partialsums for each set; conjugation means, operatively attached to the totalsum adding means to receive the total sum of the second set of partialsums, for calculating the conjugate of the received total sum;multiplier means, operatively attached to the conjugation means andtotal sum adding means to receive the total sums for multiplying thereceived total sums thereby providing a correlation function; and afrequency error estimator, operatively attached to the multiplier meansto receive the correlation function, for extracting the frequency errorfrom the correlation function.

[0020] In one embodiment the interval defining means is a sampler andadditionally there is a selective sampler connecting the partial sumadding means and the total sum adding means for selectively providingthe total sum adding means with the partial sum adding means. In otherembodiments the scaling means include means to dividing each total sumby its magnitude, and are ideal scalers. In another embodiment there isa second scaling means, connecting the multiplier means to the diversitycombining means, to receive the multiplied total sums, for scaling thereceived multiplied total sums, and providing the scaled multipliedtotal sums to the diversity combining means.

[0021] In other embodiments the frequency error estimator includes asplitter for separating the real and imaginary component of thecorrelation function to provide the imaginary component of thecorrelation function as the frequency error.

[0022] The interval defining means include the partial sum adding meanswith symbols from adjacent first and second intervals in the same set,or alternatively with symbols from interleaved sets of first and secondintervals. In each of these cases intervals can be one symbol in length,or they can be a half symbol in length.

[0023] In other embodiments, the above frequency discriminator includesa negator, that connects the partial sum adding means to the total sumadding means, for multiplying the second partial sum of each set by −1,and provides the negated second partial sum to the total sum addingmeans.

[0024] In another embodiment, the frequency error is calculated using anegator, connecting the partial sum adding means to a second total sumadding means, for receiving the second partial sum of each set from thepartial sum adding means, for negating the second partial sum of eachset my multiplying the second partial sum by −1 and a second total sumadding means, operatively attached to the partial sum adding means toreceive the first partial sum of each set and to the negator forreceiving the negated second partial sum for each set, for calculatingsecond total sum functions for the first and second sets representingthe sum of the first partial sum and the negated second partial sum ofeach set, and for providing the conjugation means with the second totalsum of the second set for conjugation. In another embodiment thefrequency error estimator is operatively connected to the diversitycombining means to receive two correlation functions corresponding tothe output of the first and second total sum adding means, for provingthe sum of the two correlation functions as the frequency error.

[0025] Further embodiments of this aspect of the invention change thereference frequency to minimize the frequency error using a loop filter,operatively attached to the frequency discriminator to receive thefrequency error, for generating an oscillator control signal based onthe frequency error to minimize the difference between the referencefrequency and the frequency of the space time transmit diversity signaland a controlled oscillator, operatively attached to the loop filter toreceive the oscillator control signal, for generating the referencefrequency based on the oscillator control signal. In further embodimentsthe controlled oscillator is a numerically controlled oscillator or avoltage-controlled oscillator.

[0026] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

[0028]FIG. 1 is a prior art block diagram of an STTD encoder;

[0029]FIG. 2 is a prior art modulation pattern for an STTD signal;

[0030]FIG. 3 is a block diagram that illustrates an AFC at UE;

[0031]FIG. 4 is a block diagram that illustrates one embodiment of themethod of operating the FD in an AFC at UE;

[0032]FIG. 5 is a block diagram that illustrates one embodiment of theFD apparatus operated on by the method of FIG. 4;

[0033]FIG. 6 illustrates an S-curve for the error signal provided by themethod and apparatus of FIGS. 4 and 5;

[0034]FIG. 7 is a block diagram that illustrates one embodiment of themethod of operating the FD in an AFC at UE;

[0035]FIG. 8 is a block diagram that illustrates one embodiment of theFD apparatus operated on by the method of FIG. 7;

[0036]FIG. 9 illustrates an S-curve for the error signal provided by themethod and apparatus of FIGS. 7 and 8;

[0037]FIG. 10 is a block diagram that illustrates one embodiment of themethod of operating the FD in an AFC at UE;

[0038]FIG. 11 is a block diagram that illustrates one embodiment of theFD apparatus operated on by the method of FIG. 10;

[0039]FIG. 12 illustrates an S-curve for the error signal provided bythe method and apparatus of FIGS. 10 and 11;

[0040]FIG. 13 is a block diagram that illustrates one embodiment ofadditional steps in the method of operating the FD in an AFC at UE; and

[0041]FIG. 14 is a block diagram that illustrates one embodiment of theadditional FD apparatus operated on by the method of FIG. 13.

DETAILED DESCRIPTION

[0042] Generally, the present invention provides a method and system fordetermining the frequency error between a reference frequency and thefrequency of an STTD signal. Further embodiments of the inventionprovide a method and system for minimizing the frequency error.

[0043] Due to the limited frequency accuracy of the frequency referencetypically used in UE, closed-loop automatic frequency control (AFC) isdesired.

[0044]FIG. 3 illustrates a block diagram of an AFC loop. The referencefrequency is a generated by a controlled oscillator (CO) 10, such as avoltage or numerically controlled oscillator. The frequencydiscriminator (FD) 20 detects the magnitude and sign of a frequencyerror that the reference frequency may have with respect to a receivedsignal. This frequency error is represented as error signal, e(t), 30and is based on the frequency offset between the frequency reference andthe received signal. The frequency error is then filtered by loop filter(LF) 40 to produce a correction signal applied to the CO to compensatefor the error. In the receiver, after despreading the channel, thecomponent received by antenna 1 is typically a phase-shifted stream ofA's. The component received by antenna 2 is an independently phaseshifted version of a stream of a “+A −A” pattern. If the frequency erroris non-zero, the symbols received by antenna 1 and antenna 2 arestatistically frequency shifted, i.e. a rotation in one direction on acomplex plane. This frequency rotation can be detected by a correlationof the samples with time difference τ.

[0045] A first embodiment of a method and apparatus for frequencydiscrimination will be described in reference to FIGS. 4-6.

[0046] In reference to FIG. 4, one embodiment of a method of frequencydiscrimination to produce a frequency error is illustrated. A first andsecond sequence of symbols, representing the STTD signal, is received,and a first and second intervals are defined in the stream. The secondinterval 250 is defined using the properties of the second sequence,which corresponds to the message transmitted by antenna 2. The secondinterval 250 is defined as the interval where the symbols received arethe same in magnitude as the symbols received in the first interval 240,but differ in their sign. A first partial sum 220A is taken as the sumof the symbols in the two sequences during the first interval 240A, anda second partial sum 230A is taken as the sum of the symbols in the twosequences during the second interval 250A. Upon calculating the partialsums, a total sum, referred to as p(t) 210A, is calculated by adding thetwo partial sums. A second total sum, also referred to as a delayedsignal, is calculated in the same manner during a second interval, andis represented by p(t−τ) 210B, where the time difference between thesets of intervals is τ 260. All references in FIG. 4 referring to thesecond set of intervals are denoted using the same numerals as those forthe first set, but are appended by the letter ‘B’ instead of ‘A’.

[0047] This method of computing the total sum 210 allows the propertiesof the antenna 2 sequence to statistically cancel the interference thatthe antenna 2 signal would have had on the antenna 1 signal.

[0048] As mentioned earlier, the delayed signal p(t−τ) 210B iscalculated in a manner analogous to signal p(t) 210A at one time periodτ 260B prior to time t. Note that FIG. 4 illustrates the invention byway of example only. As such, in the case of delayed signal p(t−τ) 210Bthe second interval 230B occurs chronologically after the first interval220B, whereas in the case of signal p(t) 210A, the second interval 230Aoccurs chronologically before the first interval 220A. A person skilledin the art can appreciate that the precise number or order of intervalscan vary, as it is dependent on the actual antenna 1 and antenna 2symbol sequences used, and that the invention can readily be adapted tomany such symbol sequences, although not expressly shown in thedrawings.

[0049] After obtaining p(t) and p(t−τ), a correlation of the twofunctions is taken. In a presently preferred embodiment the correlationis calculated by taking an average over time of p(t)p*(t−τ), wherep*(t−τ) is the conjugate of p(t−τ) as will be understood by one of skillin the art. One of skill in the art will readily appreciated that thefrequency error can be calculated in a number of ways, and that apresently preferred embodiment is to take the imaginary component of thecomplex number representation of the correlation.

[0050] Referring to FIG. 5, a multi-finger apparatus 500 that embodiesFD 20 of FIG. 3, employing the above method, can be constructed toprovide the frequency error. The multi-finger structure 500 providestime diversity by having each finger 510 provide a partial correlationwhich are then combined by an adder 310 which averages the partialcorrelations, each of which is scaled 270, thereby providing diversitycombining means. The operation of a single finger 510 will be describednext.

[0051] The received symbols from the two sequences will be stored in amemory 205, and will be divided into first and second intervals by aninterval defining means. A set of adders, for instance found indespreader 207, serving as a partial sum adding means, will add thesymbols in each of the first and second intervals to provide first andsecond partial sums. The associated first (selectively available at tap340A) and second (selectively available at tap 350A) partial sums willthen be added together to produce a total sum, the output of adder 210A.This can be in parallel, or in series with the calculation of the timedelayed partial sums (selectively at tap 350B and 340B respectively),which can be expressed as the partial sums of a second set of intervals.The time delayed total sum, the output of adder 210B, is provided to aconjugator 290, which provides the conjugate of the time delayed totalsum. The first total sum, and the time delayed total sum are then scaledby scalers 280A and 280B, which are preferably ideal or exact scalers,and are then multiplied to each other by combiner 300. Scale block 270,to scale the resulting product of the multiplication, is not needed ifthe scalers 280A and 280B are ideal magnitude or exact normalizers. In apresently preferred embodiment the scale function is defined as${{scale}(z)} = {\frac{z}{z}.}$

[0052] The selective sampler 330 can be designed to sample at some orall the intervals at which the antenna 2 component in the delay linehave opposite sign in the first 340A and second 350A taps, and oppositesign in the third 350B and fourth 340B taps. Thus selective sampler 330only provides symbols to the adders if there are identified first andsecond intervals, but the selective sampler can be designed to notprovide all such instances.

[0053] When ideal scalers are used for the second 280A and third 280B“scale” blocks, the detector S-function, an embodiment of which isillustrated in FIG. 6 having chip rate of 3.84 Mcps is defined as:${e(t)} = {{{Im}\left( \frac{{p(t)} \cdot {p\left( {t - \tau} \right)}^{*}}{{{p(t)}} \cdot {{p\left( {t - \tau} \right)}}} \right)} = {{\sin \left( {\angle \left\lbrack {{p(t)} \cdot {p\left( {t - \tau} \right)}^{*}} \right\rbrack} \right)} = {{\sin \left( {2\quad \pi \quad \Delta \quad f\quad \tau} \right)}.}}}$

[0054] Referring to FIG. 6, the control range 32, illustrated by therange of the S-function curve, indicates that a frequency errordetection range of less than 8 kHz is provided.

[0055] To increase the control range, a second embodiment of the methodand apparatus is provided and will be described in reference to FIGS.7-9. The block diagram of FIG. 7 illustrates a second embodiment of themethod. This second embodiment changes the correlator by alternating thetap order thereby increasing the control range. The signal p(t) 210A isprovided by the total sum of two partial sums, a first partial sum 220Aand a second partial sum 230A. The first partial sum 220A is the sum ofthe symbols in first interval 240A, while the second partial sum 230A isthe sum of the symbols in second interval 250A. The delay between signalp(t) 210A and p(t−τ) 210B is τ 260, which is half the value of thecorresponding delay 260 in FIG. 4. This shorter delay is due to theinterleaving of the intervals of the two sets. The interleaving of theintervals is done such that the first interval of one set is adjacent tothe first interval of the opposite set, and the second intervals of thetwo sets are adjacent to each other, resulting in a pattern of firstinterval of the first set, second interval of the second set, secondinterval of the first set, and first interval of the second set. Themethod remains the same, save for the interleaving of the intervals.

[0056] The corresponding system to this method is illustrated in theapparatus of FIG. 8. The inputs of the adders of the total sum addingmeans allow the reorganization of the intervals as described in themethod. The selective sampler 330 samples only at intervals at which theeven taps (and odd taps) have opposite sign in the antenna 2 component,to ensure that the adders are provided only with symbols correspondingto first and second intervals. The selective sampler 330 can eithersample during all such intervals, or only sample at some of suchintervals. The S-function of this embodiment is illustrated in FIG. 9,which shows that the control range 32 has been doubled in this secondembodiment and indicates that a frequency error detection range of lessthan 16 kHz is provided.

[0057] To increase the detection range further, a third embodiment willbe described in reference to FIGS. 10-12. The third embodiment uses aspecial spreader 25 that splits the 256-chip pilot symbols into two halfsymbols of 128 chip, so that the first interval 240 and second interval250 are half a symbol, or 128 chip, long. The selective sampler 330samples only during intervals in which the first tap 340A and second tap340B contain half symbols in which the antenna 2 components are oppositein sign to those in respective third tap 350A and fourth tap 350B. Thesampler 330 is restricted to sampling in these intervals, but is notrequired to sample at all these intervals. It should be noted that themethod and system for this embodiment are the same as the previousembodiments, save for the smaller symbol size. This refinement can beemployed in the apparatus and method of either the first or secondembodiments.

[0058] Having taught how to eliminate the antenna 2 interferencecomponent with respect to antenna 1 for the purposes of AFC operation,an improvement applicable to all of the aforementioned embodiments willnow be presented. The improvement makes it possible to independentlyeliminate the antenna 1 interference component with respect to antenna2, thereby providing a second frequency error signal. The two frequencyerror signals can then combined to provide a third error signal therebymaking use of the diversity gain provided by an STTD signal.

[0059] With some additional steps and apparatus, to be described belowin reference to FIGS. 13-14, it is possible to eliminate the antenna 1component in a manner analogous to how the antenna 2 component waseliminated as described in the above embodiments. What will be describedapplies equally well to any of the three embodiments described above,but for the sake of brevity will only be described in reference to thefirst embodiment, as adaptation to the other two would be obvious to aperson skilled in the art.

[0060] As compared to the method of FIG. 4, the method of FIG. 13provides additional steps to eliminate the antenna 1 component, whichcan be used in conjunction with previous methods, or on its own as aseparate error determining method. Partial sums are taken in each of thefirst and second intervals as in previous embodiments. Where previousembodiments had summed the partial sums to eliminate the secondsequence, this embodiment takes the additive inverse of the secondpartial sum, so as to eliminate the first sequence. This can beimplemented using either subtractors, or a negator, designed to negate apartial sum by multiplying by −1, in series with an adder. The total sumof the first partial sum and the negated second partial sum is q(t) 410A. In an analogous manner, the delayed signal q(t−τ) 410B is provided.The delay between signal q(t) 410A and q(t−τ) 410B is τ 260.

[0061] By providing total difference 410 in the invention, statisticallythe antenna 1 component in the first interval 240 cancels the antenna 1component in the second interval 250 thereby the antenna 1 componentthat would have traditionally interfered with the antenna 2 component inFD 20 operation is eliminated by the invention.

[0062] An apparatus adapted to allow the additional steps of eliminatingthe antenna 1 components is illustrated in FIG. 14. The apparatuspositively sets out the additional hardware required to independentlyeliminate the antenna 1 component, and although not expressly shown inthe drawing is meant to be operated in conjunction with apparatus thatindependently eliminates the antenna 2 component. Instead of using afirst adder 210A and a second adder 210B as was the case for eliminatingantenna 2 components, the apparatus for eliminating antenna 1 componentsuses a first subtractor 510A and a second subtractor 510B, or asdescribed above, it can use a negator on one of the partial sums priorto adding to obtain an additive inverse. By using systems thatindependently eliminate the interference caused by the first and secondsequences it is possible to create a third error signal.

[0063] Hence while the summation for p(t) 210A and p(t−τ) 210Beliminates the signal from antenna 2 (assuming infinite channelcoherence time), q(t) 410A and q(t−τ) 410B eliminates the signal fromantenna 1. Both correlation products p(t)p*(t−τ) and q(t)q*(t−τ) areproportional to the magnitude of the carrier-to-interference ratio (CIR)squared and sin (wt). Hence we can just add these together forming anerror signal for the AFC loop of${e(t)} = {\frac{{Im}\left( {{{p(t)}{p^{*}\left( {t - \tau} \right)}} + {{q(t)}{q^{*}\left( {t - \tau} \right)}}} \right)}{{{{p(t)}{p^{*}\left( {t - \tau} \right)}}} + {{{q(t)}{q^{*}\left( {t - \tau} \right)}}}}.}$

[0064] To further illustrate this, let g be the complex channel gainfrom antenna 1 to the receiver and let b be the gain from antenna 2.Assume that b and g are constant for the moment. Then we${have}\quad \begin{matrix}{{p(t)} = {2g\quad {A(t)}^{j\quad w\quad t}}} \\{{{q(t)} = {2b\quad {A(t)}^{j\quad w\quad t}}},}\end{matrix}$

[0065]  where “w” is the frequency error. Hencep(t)p^(*)(t − τ) = 4g²A(t)A^(*)(t − τ)^(j  w  t)q(t)q^(*)(t − τ) = 4b²A(t)A^(*)(t − τ)^(j  w  t)

[0066]  since A(t)=A(t−τ) assuming appropriate despreading, thenA(t)A*(t−τ)=1 and we have e(t)=sin (wt)

[0067] This is the same error signal as in the earlier embodiments.However, a limitation of the previous embodiments is that if thepropagation path from the second RNS antenna to the UE is severed thenno error signal for frequency tracking is available. With theimprovement outlined above, the error signal is always available to theUE unless both first and second antenna propagation paths from the RNSto the UE are severed, which would result in the loss of all symbols,rendering the loss of frequency error estimates meaningless.

[0068] As another enhancement to the above embodiments e(t) can bemultiplied by the average signal to noise ratio of the pilot signal,which results in a well controlled error signal. Hence, the AFC willhave the properties of a first order Kalman filter that compensates thefrequency control strongly when SNR is high and weakly when the SNR islow.

[0069] The embodiments of the above-described invention provide threefrequency error estimates that allow a STTD signal to be tracked in anAFC. The first set of embodiments uses the properties of the secondsequence to remove interference that the second sequence causes in thefirst sequence, and provides a frequency error estimate based on thefirst sequence. The second set of embodiments uses the properties of thefirst sequence to remove interference that the first sequence causes inthe second sequence, and provides a frequency error estimate based onthe second sequence. A third frequency error estimate is providedthrough the summation of the two previous error estimates, and providesa greater range of frequency error control than either of the first twoindependently. Additionally, in operation the third error estimateprovides additional robustness, by maintaining an frequency errorcalculation in the event of one transmitting antenna failing. Thisallows for a level of redundancy due to the dual sequences transmittedby STTD antennae.

[0070] The above-described embodiments of the present invention areintended to be examples only. Those of skill in the art may effectalterations, modifications and variations to the particular embodimentswithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

We claim:
 1. A method of obtaining a frequency error estimate of thedifference between a reference frequency and the frequency of a spacetime transmit diversity signal from first and second received sequencesof symbols, transmitted respectively by first and second antennae, whereeach sequence has two sets of first and second intervals, such that thecontents of the second interval of the second received sequence are theadditive inverse of the contents of the first interval of the secondreceived sequence, the method comprising the steps of: receiving thefirst and second sequences of symbols; calculating two sets of first andsecond partial sums as the sum of the contents of the first and secondintervals, respectively, for each set; calculating total sum functionsfor the first and second sets by summing the first and second partialsums for each set; calculating a correlation function based on the totalsum functions for the first and second sets; and extracting thefrequency error estimate from the correlation function.
 2. The method ofclaim 1, wherein the correlation function is calculated as a timeaverage of the product of the first total sum function and the conjugateof the second total sum function.
 3. The method of claim 1, wherein thereceived symbols are represented by complex numbers.
 4. The method ofclaim 3, wherein the step of extracting includes isolating the imaginarypart of the correlation function as the frequency error estimate.
 5. Themethod of claim 1, wherein the first and second interval in each set areadjacent.
 6. The method of claim 1, wherein the first and second sets ofintervals are interleaved with each other.
 7. The method of claim 1,wherein the contents of the first and second intervals in each set forma complete symbol.
 8. The method of claim 1, wherein the contents of thefirst and second intervals in each set form a half symbol.
 9. The methodof claim 1, wherein prior to the step of calculating the total sumfunctions the second partial sum for each set is multiplied by −1. 10.The method of claim 1, wherein the step of extracting includes addingthe correlation to a correlation of a second set of total sum functionscalculated by summing the first partial sum with the additive inverse ofthe second partial sum.
 11. The method of claim 1, further comprisingthe step of multiplying the frequency error estimate by the average of asignal-to-noise-ratio of the received sequences.
 12. The method of claim1, further comprising the step of altering the reference frequency basedon the frequency error estimate to minimize the difference between thereference frequency and the frequency of the space time transmitdiversity signal.
 13. The method of claim 1, further comprising thesteps of: carrying out the first four steps in parallel to provide amultitude of diverse correlation functions; and combining the multitudeof diverse correlation functions to provide the correlation functionbefore extracting the frequency error from the correlation function. 14.An apparatus having a frequency discriminator for obtaining a frequencyerror estimate of the difference between a reference frequency and thefrequency of a space time transmit diversity signal from first andsecond received sequences transmitted respectively by first and secondantennae, and received by a receiving antenna, where each sequence hastwo sets of first and second intervals, of equal length, such that thecontents of the second interval of the second received sequence are theadditive inverse of the contents of the first interval of the secondreceived sequence, the frequency discriminator comprising: a memory,operatively attached to the receiving antenna for storing the contentsof the first and second sequences; interval defining means, operativelyattached to the memory to receive the first and second sequences ofsymbols, for dividing the received sequences into sets of first andsecond intervals; partial sum adding means, operatively attached to theinterval defining means to receive the contents of first and secondsequences during the two sets of first and second intervals, forcalculating two sets of first and second partial sums as the sum of thecontents of the first and second intervals respectively for each set;total sum adding means, operatively attached to the partial sum addingmeans to receive the two sets of first and second partial sums, thetotal sum adding means for calculating total sum functions for the firstand second sets representing the sum of the first and second partialsums for each set; conjugation means, operatively attached to the totalsum adding means to receive the total sum of the second set of partialsums, for calculating the conjugate of the received total sum;multiplier means, operatively attached to the conjugation means andtotal sum adding means to receive the total sums for multiplying thereceived total sums thereby providing a correlation function; and afrequency error estimator, operatively attached to the multiplier meansto receive the correlation function, for extracting the frequency errorfrom the correlation function.
 15. The apparatus of claim 14, furthercomprising scaling means, operatively attached to the total sum addingmeans to receive the total sum of the first set of partial sums and tothe conjugation means to receive the conjugate of the total sum of thesecond set of partial sums, for scaling the received total sums.
 16. Theapparatus of claim 14, further comprising multiple frequencydiscriminators sharing a single frequency estimator, and diversitycombining means, operatively attached to the multiplier means of eachfrequency discriminator to receive the multiplied total sums, forcombining the received multiplied total sums to provide a diversecorrelation function used as the correlation function by the singleshared frequency error estimator.
 17. The apparatus of claim 14, furthercomprising scaling means, operatively attached to the multiplier meansto receive the output of the multiplier means, for scaling thecorrelation function.
 18. The apparatus of claim 14, wherein theinterval defining means is a set of samplers.
 19. The apparatus of claim14, further comprising a selective sampler connecting the partial sumadding means and the total sum adding means for selectively providingthe total sum adding means with the partial sum adding means.
 20. Theapparatus of claim 14, wherein the scaling means includes means fordividing each total sum by its magnitude.
 21. The apparatus of claim 14,wherein the scaling means includes an ideal scaler.
 22. The apparatus ofclaim 14, wherein the scaling means includes a signal to noise ratioscaler.
 23. The apparatus of claim 14, further comprising a secondscaling means, connecting the multiplier means to the diversitycombining means, to receive the multiplied total sums, for scaling thereceived multiplied total sums, and providing the scaled multipliedtotal sums to the diversity combining means.
 24. The apparatus of claim14, wherein the frequency error estimator includes a splitter forseparating the real and imaginary component of the correlation functionfor providing the imaginary component of the correlation function as thefrequency error.
 25. The apparatus of claim 14, wherein the intervaldefining means includes means for providing the partial sum adding meanswith symbols from adjacent first and second intervals in the same set.26. The apparatus of claim 14, wherein the interval defining meansincludes means for providing the partial sum adding means with symbolsfrom interleaved sets of first and second intervals.
 27. The apparatusof claim 14, wherein the interval defining means includes means forproviding intervals one symbol in length.
 28. The apparatus of claim 14,wherein the interval defining means includes means for providingintervals that are a half symbol in length.
 29. The apparatus of claim14, further comprising a negator, connecting the partial sum addingmeans to the total sum adding means, for receiving the second partialsum of each set from the partial sum adding means, multiplying thesecond partial sum of each set by −1, and for providing the negatedsecond partial sum to the total sum adding means.
 30. The apparatus ofclaim 14, further comprising: a negator, connecting the partial sumadding means to a second total sum adding means, for receiving thesecond partial sum of each set from the partial sum adding means, fornegating the second partial sum of each set by multiplying the secondpartial sum by −1; and a second total sum adding means, operativelyattached to the partial sum adding means to receive the first partialsum of each set and to the negator for receiving the negated secondpartial sum for each set, for calculating second total sum functions forthe first and second sets representing the sum of the first partial sumand the negated second partial sum of each set, and for providing theconjugation means with the second total sum of the second set forconjugation.
 31. The apparatus of claim 30, wherein the frequency errorestimator is operatively connected to the diversity combining means toreceive two correlation functions corresponding to the output of thefirst and second total sum adding means, for providing the sum of thetwo correlation functions as the frequency error.
 32. The apparatus ofclaim 14, further comprising: a loop filter, operatively attached to thefrequency discriminator to receive the frequency error, for generatingan oscillator control signal based on the frequency error to minimizethe difference between the reference frequency and the frequency of thespace time transmit diversity signal; and a controlled oscillator,operatively attached to the loop filter to receive the oscillatorcontrol signal, for generating the reference frequency based on theoscillator control signal.
 33. The apparatus of claim 32, wherein thecontrolled oscillator is a numerically controlled oscillator.
 34. Theapparatus of claim 32, wherein the controlled oscillator is avoltage-controlled oscillator.